Wollastonite is a natural calcium silicate that has a theoretical composition of CaSiO.sub.3 (which may also be written CaO.SiO.sub.2). The chemical composition of wollastonite is about 48.3% calcia (CaO) and about 51.7% silica (SiO.sub.2). One significant use of wollastonite is in ceramics such as wall tile, where it promotes low shrinkage, good strength, low warpage, and fast firing. It is also used in porcelain, as a filler in paint, plastics and papers, as an electrode coating, as a protective slag for continuous casting carbon steel and casting silicon steel sheet, and in asbestos replacement. To produce ceramics, wollastonite has been mixed with clay (an aluminum silicate) and other raw materials in various proportions to make ceramic tile. In order to make a ceramic product, for example, the wollastonite is combined with kaolin clay and nepheline syenite, which is a type of feldspar, and then fired. Sainamthip & Reed, Am. Ceram. Soc. Bull. 66(12):1726-31 (1987).
Wollastonite is a relatively rare, naturally occurring mineral, mined in New York in the United States and in various foreign countries. Because of its relative scarcity (and, therefore, high expense), synthetic methods have been devised to produce wollastonite. Such methods have included heating a mixture of silica and limestone with the addition of a fluxing agent to 1100.degree. C. for several hours. It is believed that the long heating time is required because each of the components of the mixture is present in a high melting phase in the mix and because such raw materials need to undergo several chemical steps during the reaction in which wollastonite is formed. Because of the long heating time, and therefore heightened expense, these methods of synthesizing wollastonite have not been suitable for industrial production of wollastonite.
It is believed that one of the major reasons surrounding the successful use of wollastonite in ceramic products is that silica is not present as a separate phase in the ceramic product. Silica has a significantly different thermal coefficient of expansion than the other phases present in the fired ceramic product. It also undergoes a phase transformation during the cooling of the ceramic product. Both of these factors cause cracking of the ceramic product unless a slow cooling rate is used. By eliminating the free silica phase, higher firing and cooling rates can be employed, resulting in a correspondingly higher kiln capacity.
There has gone unmet, however, a need for an inexpensive starting material that can be used to make wollastonite, or a ceramic (or other) product containing wollastonite, that requires a shortened heating time, and a shortened cooling time.